The present invention relates to flow meters for determining the flow rate of a liquid. More particularly, the present invention relates to flow meters capable of determining the corrected flow rate of a liquid based on a deterministic, closed solution.
Flow sensors are generally used to measure an uncorrected flow rate of a liquid flowing in a confined space such as a conduit like a tube or pipe. Among the types of conventional flow sensors are paddle wheel sensors in which a rotatable paddle wheel is inserted into the flow path of a liquid. The flowing liquid causes the paddle wheel to rotate and the flow rate of the liquid is determined based on the speed of rotation of the paddle wheel. Another type of conventional flow sensor is a turbine flow sensor in shich the speed at which a turbine turns in a liquid is used to determine flow rate. However, these particular types of flow sensors are generally not very accurate for measuring low flow rates or flow rates of high viscosity liquids.
Orifice flow sensors have also been used to determine the flow rate of liquid and are better suited for determining low flow rates and the flow rates of viscous liquids. An orifice flow sensor typically comprises an orifice restriction placed in the liquid flow path. The orifice restriction impedes the flow of liquid creating a pressure drop. Pressure sensors can be included for measuring the pressure drop and a flow rate of the liquid can be determined based on the pressure drop, the conduit diameter, and the orifice diameter, as well as certain properties of the liquid such as temperature, viscosity, and density. However, orifice flow sensors do not provide accurate measurements over a wide range of flow rates and liquid properties.
Flow meters have been designed which use discharge coefficients to correct inaccuracies found in conventional flow sensors. Flow meters calculate the corrected flow rate by multiplying the uncorrected flow rate, measured by a conventional flow sensor, by these discharge coefficients to determine a corrected flow rate that more accurately reflects the flow rate of the liquid. These discharge coefficients are typically dependent on liquid flow rate, viscosity and temperature of the liquid, the geometry of the liquid flow path as well as other characteristics specific to the flow meter and its particular application. This detailed information about liquid properties must be know in order to determine the discharge coefficient for a specific flow meter application. Additionally, the discharge coefficient is only accurate for a very specific flow meter configuration and application because it is dependent on the liquid being measured and the meter application properties. Thus, a new discharge coefficient must be calculated if the liquid or application changes, rendering the discharge coefficient relatively useless in a system that must adjust to a variety of liquids with different physical properties.
Corrected flow rate calculations using convention discharge coefficients are determined using recursive calculations. Recursive calculations can be very time consuming and in some situation non-deterministic. Thus, conventional discharge coefficients may not be very useful for correcting inaccurate flow rate measurements in systems in which the corrected flow rate calculations are done repeatedly at very short time intervals.
Accordingly, a need exists for an improved flow meter for accurately measuring the flow rate of a liquid flowing in a conduit. There is also a need for an improved discharge coefficient that is relatively independent of liquid properties and flow meter application characteristics. There is a further need for a discharge coefficient that can be quickly and deterministically calculated.
These needs and others are satisfied by a flow meter according to the present invention which comprises a flow sensor, a temperature sensor and a microprocessor. The flow meter is configured for determining the flow rate of liquid flowing in a fluid flow path in a confined space. As described below, the conduit diameter is used in calculating the corrected flow rate of the liquid. The liquid is categorized into a predetermined class of liquids based on its viscosity and density with the liquid category being an input to the microprocessor. Each class of liquids comprises liquids having a viscosity and density within a specific range designated for the class.
The flow sensor is configured for measuring an uncorrected flow rate of the liquid flowing in the fluid flow path. The temperature sensor is configured for measuring the temperature of the liquid. The microprocessor determines a corrected flow rate of the liquid based on the predetermined class of the liquid and the measured temperature.
The flow meter of the invention preferably employs an orifice type flow sensor which provides a restriction in the flow path, the restriction creating a second diameter in the flow path, and a differential pressure sensor for measuring the pressure drop created by the restriction. In one embodiment, the differential pressure sensor comprises first and second pressure sensors. The first pressure sensor, positioned on a first side of the restriction, is configured for measuring pre-restriction pressure created in the confined space by the liquid flowing in the fluid flow path on the first side of the restriction. The second pressure sensor, positioned on a second side of the restriction opposite the first side, is configured for measuring post-restriction pressure created in the confided space by the liquid flowing in the fluid flow path on the second side of the restriction. Alternatively, a single differential pressure sensor can be used for measuring the pressure drop.
The microprocessor is connected to the differential pressure sensor (or first and second pressure sensors) and receives the differential pressure (or pre-restriction and post-restriction pressures from the first and second pressure sensors, respectively). The microprocessor determines the flow rate of the liquid flowing in a fluid flow path in a confined space based further on the measured temperature and differential pressure. The confined space includes a first diameter and the restriction creates a second diameter. The first and second diameters dimensions are also used by the microprocessor for determining the flow rate.
Preferably, the first and second pressure sensors comprise silicon-based MEM sensors and the temperature sensor preferably comprises a thermocouple positioned in the fluid flow path. Alternatively, the temperature sensor can comprise a resistive bridge. Additionally, the flow meter can include protective membranes positioned between the first and second pressure sensors and the liquid. While specific types of flow and temperature sensors are mentioned, it can readily be appreciated that any conventional flow or temperature sensor and, in the case of orifice flow sensors, any type of suitable pressure sensor, can be used without departing from the spirit and scope of the invention as set forth in the appended claims.
In one embodiment, the corrected flow rate is determined using the following equation:       Q    u    =                    -                  a          1          xe2x80x2                    -                                                  (                              a                1                xe2x80x2                            )                        2                    -                      4            ⁢                          a              0              xe2x80x2                        ⁢                          a              2              xe2x80x2                                                  2      ⁢              a        2        xe2x80x2            
where
aoxe2x80x2=a0Qu;
a1xe2x80x2=a1Quxe2x88x921;
a2xe2x80x2=a2Qu;
a0, a1 and a2=predetermined coefficients (which are determined empirically as described in the detailed descriptions) based on the temperature and class of liquid; and
Qu is the uncorrected flow rate measured by the flow sensor.
One application of a flow meter according to the present invention is in a beverage dispensing machine for measuring the flow rate of liquid processed by the beverage dispensing machine. In a particularly preferred embodiment of the invention, the flow meter is used in a dispensing machine that mixes a flavored syrup concentrate and water, for example, carbonated water, to obtain the beverage. In this kind of beverage dispensing machine, the syrup is allowed to flow freely and, based on its flow rate, the flow rate of the water is adjusted to provide a a beverage when dispensed having the desired ratio of syrup to water. The flow meter can include or be combined with a valve for regulating the flow rate of the water based on the determined flow rate of the syrup. In a particularly preferred embodiment, a second flow meter of the present invention is used to measure the flow of the water as well in order to premit accurate adjustment of the valve used to adjust its flow relative to the freely flowing syrup. Additionally, the flow meter can be configured for controlling the amount of liquid dispensed by the syrup dispenser based on the determined flow rate.
The flow meter can be used in various other applications including with a variable speed pump to measure and control the flow rate of liquid pumped by the pump. The flow meter is configured to make periodic flow rate calculations and because it has no mechanical inertia, it is particularly suited for measuring the non-uniform flows generally found in variable speed pumps. The flow meter can include a wireless communication system for transmitting the determined flow rate to a central control center or for receiving control signals from the central control center for controlling operation of the flow sensor.
In a liquid beverage dispenser application in which the beverage dispenser is configured to mix syrup and water to produce a liquid beverage, the flow meter can be configured for determining the flow rate of syrup flowing in a syrup flow path in a syrup dispensing conduit and adjusting the flow rate of water based on the flow rate of syrup to provide a beverage having the desired ratio of syrup to water. In this application, the syrup falls into a predetermined class of syrups based on the viscosity and density of the syrup. It can be readily appreciated that flow meters according to the present invention can also be used in beverage dispenser that mix more than two liquids as long as the mixing ratio is defined.
The flow meter comprises an orifice restriction in the conduit, first and second pressure sensors, a temperature sensor, a microprocessor and a valve in the water flow path. The orifice restriction has an orifice diameter that is smaller than the conduit diameter The first pressure sensor, positioned on a first side of the orifice restriction, is configured for measuring pre-restriction pressure created in the conduit by the syrup flowing in the syrup flow path on the first side of the orifice restriction. The second pressure sensor, positioned on a second side of the orifice restriction, measures the post-restriction pressure created in the conduit by syrup flowing in the syrup flow path on the second side of the orifice restriction. The temperature sensor measures the temperature of the syrup.
The microprocessor is connected to the temperature sensor, and the first and second pressure sensors for receiving the measured temperature, and the pre-restriction and post-restriction pressures from the temperature sensor, first pressure sensor and second pressure sensor, respectively. The valve is connected to the microprocessor for regulating the flow rate of the water. The microprocessor is configured to actuate the value to adjust the flow rate of water based on the determined flow rate of the syrup.
Further objects, features and advantages of the present invention will become apparent from the following description and drawings.